The central subsystem of a computer may include various removable units in the form of electronic boards connected electrically to a bus, typically known as a "bottom-of-the-rack bus", via a connector. Such a connector has the particular advantage of facilitating maintenance operations when a defective board is to be replaced. Naturally the same system may have a plurality of buses. The bus to which certain units of the system are connected generally includes functional lines that carry digital signals among the boards. It also includes supply lines and maintenance lines that transmit supply and maintenance signals. The functional bus lines furnish the connections among the transmitters and receivers of the board connected on this bus. A predetermined functional line can thus be connected to a plurality of transmitters and a plurality of receivers belonging to different boards. When the system is in operation, this means that the connection or disconnection of one of the boards of the system threatens to interfere with signals exchanged on the bus.
Spurious signals may be engendered by line effects such as connector contact rebound. Furthermore, the state of the transmitters of a board being manipulated is uncertain, and the transmitters may emit undesirable signals. Moreover, connection of a board may cause interference due to current surges upon connection to a source voltage.
Currently, most electrical systems, and in particular data processing systems, are modular, to permit expansion in terms of power or memory capacity. For example, a central computer subsystem includes a plurality of processor boards, a plurality of memory boards, and a plurality of input-output monitor boards. This modular technique also makes it possible to operate the system in a downgraded mode in the case of failure of one or more boards.
However, when the failure of a board has been detected, the maintenance service must intervene to replace it. This operation is generally performed with a maintenance device that includes a service processor and acts on the processing units and memories in such as a way to safeguard the data if possible and isolate the board to be replaced. The system is then stopped during the replacement operation. The maintenance agent then commands the service processor to restart the system.
Although this method guarantees that the manipulations of the board will not cause electrical interference, leading to wrong information in the system, it requires that system operation be completely interrupted for an indeterminate period of time. This interruption is typically accompanied by the definitive stoppage of the programs being run; consequently these programs must be rerun completely after reinitialization of the system. This causes major problems for users. Moreover, the interruption in system operation may be unacceptable for applications that require absolute availability of the system. As a result, expensive redundant or parallel systems must be provided.